Crystal cutter driving circuit



Feb. 13, 1951 L. v. WELLS 2,541,393

CRYSTAL CUTTER DRIVING CIRCUIT Filed Jan. 22, 1947 I hN A m P. 11

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INVENTOR.

LAWRENCE \a'. WELLS.

ATTORNEYS'- Patented Feb. 13, 1951 CRYSTAL CUTTER DRIVING CIRCUIT Lawrence V. Wells, Charlotte, Mich, assignor to Wilcox-Gay Corporation, Charlotte, Mich., a corporation of Michigan Application January 22, 1947, Serial No. 723,535

3 Claims. (Cl. 179100.4)

This invention relates in general to the field of signal recording, and more particularly to a novel crystal recording system featuring efficient low power operation with excellent overall quality and response.

In the recording of sound upon discs or the like, a piezoelectric element, such as crystal, is often utilized for the conversion of electrical signal to be recorded into corresponding mechanical vibrations. A crystal is essentially a voltage operated device, and the mechanical forces developed thereby are transferred to a suitable cutting stylus which in turn engages the record disc. In conventional sound recording systems, a mechanical drive is arranged whereb the stylus traces a spiral path over the record disc surface, recording the signal in a groove of predetermined depth and spacing.

Ordinarily the signal to b recorded is obtained as the amplified output of a microphone or radio tuner. In order to obtain a recording which is in accord with general commercial practice, it is often essential that the signal be modified to conform with an accepted response curve. For example, it is necessary t limit th maximum cutting stylus deviation to preclude interference and actual cutting into an adjacent groove. This problem is of prime importance at the low operating frequencies, and accordingl it is desirable to supply a crystal cutter with a substantially constant driving voltage'for all frequencies below a selected value, known as the crossover frequency. For driving frequencies higher than the crossover frequency it is desirable to have the driving voltage be an inverse function of the driving signal frequency; a voltage drop of approximately 6 db. per octave (one-half voltage per octave) being a generally accepted standard.

The crossover frequenc of a recording system, that is, the frequency below which recording is accomplished at constant amplitude and above which at decreasing amplitude,- is generally selected within th low audio frequency range between 250 and 1000 cycles per second. The precise frequency in this range is not critical.

Heretofore, it has been the practice to drive a crystal recording system from an amplifier stage having considerable power capacity, such as a power pentode, or push-pull power pentodes, notwithstanding the known fact that the power requirement of a crystal is of the order of one half watt or less, for successful transcription. High power output circuits have been thus used in order to obtain a recording response characteristic relative to the selected crossover frequency, havpractice of utilizing high power circuits in recording units placed a severe limitation on design features such as portability, compactness and power drain.

The present invention contemplates and has as a primary object the provision of a recording system utilizing a crystal cutter and driving amplifier which delivers the required amount of power directly to the crystal substantially without loss other than inherent tube circuitdissipation, and which provides the aforementioned frequency response with minimum distortion.

In accomplishing the feature of the foregoing object, the following considerations are noted. A recording crystal presents an electrical impedance which is predominatel capacitive, although the mechanical record cutting accomplished thereby may be represented b a small resistive component. If a crystal element is placed in series with a resistance equal to the crystal reactance at the crossover frequency selected," and the series combination driven from a recording signal source of zero impedance, then a voltage frequency recording characteristic having substantially the desired features, mentioned above, will be obtained. This will be set forth in greater detail below.

Generally, in accordance with the present invention, the crystal cutter is transformer coupled directly to the plate of a low power, voltage amplifier type electron tube. If the plate resistance of the output electron tube is selected to equal the crystal capacitive reactance at the crossover frequency, then with a 1:1 turns ratio transformer, the required frequency response is obtained, and it will be noted that other than inherent tube losses, and generally insignificant transformer losses, no unnecessary power need be generated and no elaborate frequency compensating circuits need be included. As the entire tube output power is delivered to the crystal, a voltage amplifier triode having low maximum power output and correspondingly low power consumption is ideally suited.

As will be more clearly described below, the turns ratio of the coupling transformer may be selected to provide proper frequency response for tubes where the plate resistance is not precisely equal to the crystal reactance at the crossover frequency. Furthermore, the turns ratio of the transformer may be readily selected to provide a combination with an available tube and crystal a desired crossover frequency.

It is therefore another object of the present invention to provide a crystal sound recording system utilizing a low power voltage amplifier electron tube as an output stage.

A further object of the present invention is: to provide a recording system wherein a crystal is driven effectively in series with the plate resist ance of an electron tube.

Another object of the present invention is to provide a recording circuit utilizing a crystal cutter driven through a. transformer operative to establish a predeterminedcrossover frequency-..

A still further object of the present invention is the provision of a recording circuit particularly adaptable to lightweight portable construction and requiring comparatively littlepower for operation.

Still another object of. this invention is to provide a recording circuit of maximum simplicity using a minimum number of components.

These and other objects of the present invention will now become apparent from the follow-- ing detailed specification taken in connection with the accompanying drawings inv which:

Figure 1 is a schematic circuit diagram generally illustrating one embodiment of the present invention,

Figure 2- is a schematic diagram of the equivalent circuit of Figure 1,

Figure 3 is a still further simplified equivalent circuit of Figure 1 and.

Figure 4 is a graphical. representation of ideal and. actual recording response characteristics.

With reference now to the drawings and more particularly to Figure 1, there is: illustrated a recording system embodying the features of the present invention. The recorder shown utilizes a crystal element. II to which is secured a cutting stylus l2, in a manner well known. in the art. The cuttingv stylus I2v is engaged with a recording disc [3, supported upon turntable l4 driven at a suitable speed by motor 15. As the recording progresses, means not herein illustrated are: em ployed to displace the stylus I 2 radially of record disc [3 so that a spiral groove is cut.

The basic mechanical system components are illustrated in Figure l to facilitate understanding of the present invention; but is no: limitation thereof, since crystal recording systems are utilized widely with other mechanical. drives and recording media.

Recording crystal H is energized. by the ampli fied signal output of a voltage amplifier type electron tube It. As illustrated the plate of tube !6 is connected to a. positive voltage source B+ through the primary H of an audio type transformer 18. The crystal H is shunted directly across and provides the sole electrical load for the secondary winding IQ of transformer 48. The turns ratio of the transformer is shown in Figure 1 as 1:N' from primary to secondary.

The cathode of electron tube 16 is returned to ground through a conventional resistancecapacitance bias circuit 2 l. The control grid of tube I 6 is energized by a signal voltage e which is the output of an amplifier 22, used if required,

to raise the level of the signal to be recorded; in this example the output of microphone 23. Clearly, the circuit shown may be used to record any suitable signal lying in the frequency band for which the circuit is designed.

For purposes of analysis, the basic circuit of Figure 1 may be represented by an equivalent circuit, such as shown in Figure 2. In Figure 2, the vacuum tube It has been replaced by its functional equivalent; namely, a generator 25 of zero internal impedance and signal voltage e where u is the amplification factor of the tube it; in series with a resistance Tp which is the plate resistance of tube it. This representation is well lmown in the electronic art and no derivation thereof need he presented. As microphone 23 and amplifier 22. form no part of the present invention, they have not been shown in the equivalent circuit, Figure 2.

The equivalent circuit of Figure 2 concerns itself solely with the alternating signal voltage e and thus, the primary winding ll. of transformer is is shown directly across the series coinbi-nationof generator 25' and plate resistance r The crystal l! and its associated cutting stylus are shown in the. transformer. secondary circuit as in Figure 1..

For further simplification, the equivalent. circuit of Figure 2 has beenreduced. to components as seen from the. secondary side of transformer 1-8, in the. circuit of Figure 3.. As is well known, a primary signal voltage may be represented in the secondary circuit if multiplied by the factor N, the turns ratio... Thus, in. Figure 3,. the signal generator 3!. is. represented as having an output voltage of EX le In a. similar manner a primary circuit impedance may be transformed into. the secondary circuit if multiplied by a factor N the square of the turns ratio. 'Thus, in Figure 3,, the the resistance in series with generator is represented by N r This series circuit is thus shunted directly across. the crystal cutter l l.. It is recognized. that the simplified circuit of Figure. 3 is attainable only if the transformer 1 .8, Figure 1, is assumed to be ideal in its characteristics, which means that transformer winding resistance andl'eakage are negligible. This assumption is valid since audio transformers are available, which except for the extreme high and low frequencies, have such ideal characteristics.

Continuing the analysis of the novel circuit of this invention in terms of its equivalent, Figure 3, it will be noted that the impedance of a crystal cutter is predominantly capacitive, represented in Figure 3' by the symbol C, and having a capacitive reactance a crystal cutter as a function of frequency for cutting a record in accordance with standard practice. Below an arbitrary frequency Fe, selected as the crossover frequency for the particular system, the crystal voltage is constant. On the graph, output voltage is plotted in decibels (db.) with zero as a selected reference, and in steps of six db., which of course means that.

each step down from zero represents one half the voltage of the preceding step. Above the crossover frequency Fe, the ideal output voltage falls off at the rate of six db. per octave. Frequency is plotted in terms of octaves about the crossover frequency in Figure 4, whereby the relation of voltage decrease at six db. per octave is readily apparent.

In order to illustrate how the novel and simplified recording circuit of Figure 1 provides a characteristic substantially as shown by the ideal relation in Figure 4, reference is again made to Figure 3. For this circuit, the output voltage across the crystal e may be expressed in terms of the voltage of generator 31 by the following relation:

60=N#6 x DV F J Where 1 21rFC the reactance of the crystal l I. If now, the turns ratio of N of transformer I8 is selected so that:

at the crossover frequency, a plot of en as a func tion of F, with the value of 0=Nfl6g considered as a zero db. reference, will appear substantially as shown by the broken line on Figure 4, and closely approximate the desired curve shown by the solid line, A turns ratio N of unity may be thus used if the plate resistance of tube I6 is 'equal to the crystal reactance Xc at the selected crossover frequency.

In actual practice, the crystal reactance X0 is measured at the selected crossover frequency, and the tube plate resistance determined from the tables. The required turns ratio may then simply be found from:

and the system will have the desired characteristic.

It is apparent from the equivalent circuit of Figure 3 that the power delivered by the generator 33! is delivered in part to the recording stylus i2 and the remainder dissipated in the inherent resistance Tp of the electron tube, and that no other circuit components are present for the consumption of power. Therefore, the required characteristic is obtained with minimum power loss and circuit power input.

It is to be noted that this novel circuit because of its economy of power permits the use of a simple voltage amplifier triode type tube as tube it in Figure 1. As an example a triode tube was used with a plate voltage of +250 and with a transformer l8 having a turns ratio N of 2.7. This circuit produced recordings of higher level and less distortion than had formerly been obtained with power pentode output stages and their associated equalizing network.

It is to be emphasized that the transformer it of Figure 1 is not an impedance matching device in the usual sense where a source is employed to deliver power to a matched load. Instead the transformer I8 is employed to establish a predetermined frequency response by proper selection of its turns ratio. As the total crystal power is small, and as this is the only power transferred by transformer 3, a comparatively small transformer may be used which will not hinder production of a compact portable unit.

Although Figure 1 illustrates a conventional type isolating transformer 18 as the coupling element it is readily apparent that an autotransformer may be substituted without altering the fundamental principles involved. If an autotransformer is employed, it is desirable'to include a coupling capacitor (not shown) between the autotransformer output and the crystal and a high resistance across the crystal to preclude application of plate potential to the cutter.

It will be understood that the resistance of the winding of an actual transformer may slightly alter the frequency design consideration presented above. This resistance, if significant, may be considered as an equivalent value in series with N Tp in Figure 3. It may be desirable under certain circumstances to place a small resistance in series with the secondary winding and crystal to adjust the crossover frequency. However, maximum advantage is realized when the principal element determining the crossover frequency is the tube plate resistance.

The principles hereinabove described may be employed for recording with a push pull circuit. using two triodes such as It of Figure 1, or simply a twin triode such as tube type 6SN7. In such operation a push pull input signal is required and the transformer 18 need be modified only as are conventional push pull output transformers, that is, with a center-tapped primary for connection to 3+. The ends of the primary are simply connected to the push pull tube plates for proper operation. In push pull operation of this invention, professional standards of performance are readily attained.

For the purpose of simplifying the above specifications, the recording system has been considered as one utilizing a crystal cutter. Actually, however, the principles of this invention may be applied to recording in general, where a crystal recording head is employed. Thus, where the specifications and claims call for crystal cutter" it is to be thought of as a crystal recording unit, applicable to numerous other recording systems such as record embossing, and the like.

It is apparent that various circuit design modifications may be made by those skilled in the art without departing from the principles of this invention. Accordingly it is preferred that the spirit and scope of thisinvention be defined not by the specific disclosures hereinabove set forth, but by the appended claims.

I claim:

1. A sound recording system comprising, in combination, a voltage amplifier electron tube for amplifying signals to be recorded and having at least a cathode, grid and plate, means for impressing said signals to be recorded between said grid and cathode, an output transformer, the primary winding of said transformer being in circuit with said electron tube plate, a crystal cutter, said crystal cutter being connected across the secondary winding of said transformer, the turns ratio 1:N of primary to secondary turns on said transformer being selected to satisfy the relation:

N r =Xc wherein N is the square of the primary circuit impedance transformed into the secondary unit of the transformer 7'1) is the plate resistance of said electron tube and X0 the reactance of said crystal cutter at a predetermined crossover frequency.

2-. A crystal cutter recording system having a response wherein the voltage at said crystal cutter is substantially constant for frequencies below a selected crossover frequency and falls uniformly at the rate of six db. per octave for frequencies above said crossover frequency, said system comprising an electron tube amplifier, a transformer coupling the output of said tube to said crystal cutter, the plate resistance of said electron tube as seen from said crystal cutter being substantially equal to the reactance of said crystal cutter at said crossover frequency and efiectively in series with said crystal cutter.

3. A crystal cutter recording system having a response wherein the voltage at said crystal cutter is substantially constant for frequencies below a selected crossover frequency and falls uniformly at the rate of six db. per octave for frequencies above said crossover frequency, said system comprising a voltage amplifier triode tube, a transformer in the plate circuit of said voltage amplifier, said crystal cutter being connected to said transformer the turns ratio 1:N of primary to secondary turns on said transformer being selected to satisfy the relation:

wherein N is the square of the primary circuit impedance transformed into the secondary unit of the transformer, T1: is the plate resistance of said electron tube and X0 the reactance of said crystal cutter at a predetermined crossover frequency.

LAWRENCE V. WELLS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,906,214 Nicolson Apr. 25, 1933 1,980,888 Thomas Nov. 18, 1934 2,106,051 Dunning Jan. 18, 1938 2,218,399 Lebel Oct. 15, 1940 OTHER REFERENCES Motion Picture Sound Engineering, published by Van Nostrand Co. Inc, 1938, page 209. Copy in Div. 16.

Radiotron Designers, Handbook, by F. Langford Smith; distributed by RCA Mfg. 00., 8rd edition, 1941, page 2-14. Copy in Cong. Lib. 

